In recent years, owing to-dramatic progress in information and communication technology, the Internet has changed from a simple medium for retrieving information from media for supplying and receiving various types of information and services. In association with this change, requests for a more comfortable environment, i.e., requirements for “larger quantities of information at higher speeds, in a more real manner” have become strong. In order to realize this comfortable environment (of the Internet), it is necessary to send and receive a large amount of information in a short time and high-speed design of signals, i.e., high-frequency design of signals becomes indispensable.
A printed wiring board used in a substrate which handles the high-frequency signals requires techniques for minimizing transmission losses in order to keep and ensure the quality of the signals. The transmission losses can be divided into the three factors of conductor losses, dielectric losses and radiation losses. When the factors of the transmission losses are viewed from the standpoint of copper foil which constitutes conductor circuits of a printed wiring board, the following can be considered.
First, conductor losses will be described. In the case of high-frequency design of signals, the skin resistance of a conductor circuit increases and conductor losses increase. In a high-frequency signal, the skin effect phenomenon occurs, that is, when an alternating current is caused to flow in a conductor circuit, back electromotive force is generated in the central part of the conductor circuit due to a magnetic flux change and the current becomes less apt to flow, with the result that the phenomenon that the current flows in the surface portion of the conductor occurs. For this reason, the effective sectional area where the current flows in the conductor circuit decreases and resistance increases. This is called skin resistance. The skin portion where the current flows in the conductor circuit is called the skin depth and it is considered that this skin depth decreases inversely proportional to the square root of a frequency.
When an investigation was made into the skin depth and skin resistance of a conductor circuit at each frequency from the relationship between this skin resistance, skin depth and frequency, it became apparent that the skin depth becomes not more than 2 μm when a signal frequency becomes that of the GHz zone and that a high-frequency signal flows only in the surface layer and skin portion of the conductor circuit. Furthermore, when transmission losses were investigated by forming conductor circuits from copper foil having different surface shapes of a conductor circuit, i.e., different shapes (roughness) of a surface bonded to the substrate, transmission losses showed a tendency to increase when a conductor circuit is formed from copper foil having large roughness of the bonded surface. This is due to the skin effect of the conductor circuit and it might be thought that this is because the propagation distance of a signal becomes long in copper foil having high bonded surface roughness. From this, it might be thought that copper foil having low bonded surface roughness, i.e., low-profile copper foil is effective as the copper foil used in forming a printed wiring board for high-frequency applications.
Next, dielectric losses will be described. Dielectric losses are determined by the dielectric constant and dielectric dissipation factor of a substrate resin. When a pulse signal is caused to flow in a conductor circuit, a change occurs in an electric field around the conductor circuit. A delay in the electric field change occurs when the cycle of the change (frequency) of an electric field approaches the relaxation time of the polarization of the substrate resin (the travel time of a charged body which polarizes). And when such a state is produced, molecular friction occurs within the resin and heat is generated, resulting in losses. For this reason, in a resin used for high-frequency signals, substituents of large polarity are minimized or brought into the state of null in order to cause the above-described polarization by an electric field change to occur less easily. Incidentally, in this specification a substrate having low-dielectric constant properties in which such substituents of large polarity are minimized or brought into the state of null or a substrate having low dielectric dissipation factor properties is called a low-dielectric substrate.
Incidentally, such substituents of large polarity contribute greatly to the chemical adhesion between a substrate resin and copper foil. Therefore, in a low-dielectric substrate formed from a resin in which substituents of large polarity are minimized (or brought into the state of null) in order to lower the dielectric constant and the dielectric dissipation factor, the adhesion to the copper foil is not good and hence the peel strength of a conductor circuit tends to become very low. A comparison of peel strength between the conventionally used FR-4 substrate and a low-dielectric substrate reveals that in the FR-4 substrate, an cohesive failure (the state in which the bond is fractured at the interface within the substrate resin) occurs during peeling and high peel strength is obtained, whereas in a low-dielectric substrate, an interfacial failure (the state in which the bond is fractured between the copper foil and the substrate resin) occurs and only low peel strength is obtained. When the peel strength is low, there is a possibility that circuit peeling during the manufacturing process of a printed wiring board and the falling-off of mounted parts in the outermost layer may occur and this is undesirable.
In order to reduce the dielectric losses, it is possible to cope with a decrease in the peel strength, which occurs when a low-dielectric substrate is used, by devising copper foil. That is, it is possible to improve the peel strength by increasing the bonded surface roughness of the copper foil. However, when the bonded surface roughness of the copper foil is increased, the effect on conductor losses increases as described above although the problem of the peel strength in a low-dielectric substrate can be solved.